In general, an automatically shiftable transmission for a motor vehicle includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between an input shaft and output shaft of the transmission. The input shaft is coupled to a power source, such as an internal combustion engine through a fluid coupling, such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
It is well known that transmission shifting can be controlled by manipulating the clutch pressures and the engine torque output during the shift for improved shift quality and transmission durability. Some control strategies use open-loop engine output torque controls to suppress engine flare during low torque upshifting, while others employ closed-loop engine torque and on-coming clutch pressure controls to maintain a desired constant output torque.
Ideally, these control strategies command a step-like reduction in engine torque at the instant that the on-coming clutch gains capacity. However, in practice, these two events rarely occur at the same time. If a step engine torque reduction occurs before the on-coming clutch gains capacity, a noticeable sag in engine torque would be felt by the operator of the vehicle. Calibration engineers have deemed this result unacceptable, therefore, typical transmission calibrations require the on-coming clutch to gain capacity prior to the step torque reduction.
As transmission calibrations have continued to be refined to produce smoother shifts, larger step torque reductions and lower initial on-coming clutch pressure commands have resulted. As this trend continued, it was determined that an initial on-coming clutch pressure could be calculated and commanded to not break free or release the off-going clutch until the step torque reduction had occurred. This method of operation also results in poor shift quality as a result of tie-up between the on-coming clutch and the off-going clutch prior to the engine torque reduction.